Currently, crystalline silicon (including multi- and mono-crystalline silicon) is the most dominant absorber material for commercial photovoltaic applications, with (mono and multi) crystalline silicon modules accounting for over 80% of the photovoltaic market today. The relatively high efficiencies associated with mass-produced crystalline silicon solar cells in conjunction with the abundance of material garner appeal for continued use and advancement. But, the relatively high cost of crystalline silicon material itself (due to its dependency on polysilicon feedstock, silicon ingot growth, or cast brick formation and wafering) limits the widespread use of these solar modules. At present, the cost of “wafering”, or crystallizing silicon and cutting a wafer, accounts for about 40% to 60% of the finished solar module manufacturing cost.
As an alternative to “wafering”, methods of growing monocrystalline semiconductors, such as silicon, and releasing or transferring the grown wafer have been proposed. Yet regardless of the formation methods, a low cost epitaxial semiconductor, such as silicon, deposition process accompanied by a high-volume, production-worthy, uniform and reliable low cost method of forming a release layer or release layers are prerequisites for wider use of solar cells manufactured by semiconductor deposition and release processing.
Porous silicon (PS) formation is a fairly new field with an expanding application landscape. Porous silicon is often created by the electrochemical etching of silicon wafers with appropriate doping in an electrolyte bath. The electrolyte for porous silicon is: HF (49% in H2O typically), isopropyl alcohol (IPA) (and/or acetic acid) or other alcohols, such as ethanol, or combinations thereof, and deionized water (DI H2O). IPA (and/or acetic acid) serves as a surfactant and assists in the uniform creation of PS. Additional additives such as certain salts or acids may be used to enhance the electrical conductivity of the electrolyte, thus reducing its heating and power consumption through ohmic losses.
Porous silicon has been utilized as a sacrificial layer in MEMS and related applications, where there is a much higher tolerance for cost per unit area of the wafer and resulting product than solar PV. Typically, porous silicon is produced using simpler and smaller single-wafer electrochemical process chambers with relatively low throughputs on smaller wafer footprints—a costly and inefficient process. The viability of this technology in solar PV applications hinges on the ability to industrialize the process to large scale (at much lower cost), requiring development of very low cost-of-ownership, high-productivity porous silicon manufacturing equipment.
Designing porous silicon equipment and formation methods that allow for a high throughput, cost effective porous silicon manufacturing remains a challenge.